I don't think so. Apple has not made any information or resources available to help make more themes. The main xml file is pretty nasty looking. It will probably take a while for anyone to get a whole new theme up and running.
Yeah, the Keynote Pro guys said in their blog they've gotten a bunch of requests but the app really doesn't lend itself well to third party themes, unlike Keynote and Pages. It's really a shame since it could've turned into a booming market. Hopefully Apple opens it up and offers an iWeb theme development package or something. As customizable as the themes are, third party themes would still be really useful.